How to add schema markup, and how to check it stayed added
You can find a hundred guides for adding schema to one page and none for verifying it across a site.
Use JSON-LD, generated, never hand-written
Put a script tag of type application/ld+json in the head. Build the object in code and serialise it. Hand-writing JSON into a template is how a trailing comma silently voids every type on the page, and nothing will warn you.
Everything in the markup must be visible on the page
Marking up steps, prices or answers that a reader cannot see is the fastest route to a manual action. The markup describes the page, it does not extend it.
Then verify across the whole site, not one URL
Single-URL testers confirm one page. They cannot tell you that three of your sixty product pages lost their block in a template change, which is the failure that actually costs traffic and the one nobody checks.
Put it in the HTML, not in JavaScript
Google can execute JavaScript, but it does so on a delay and not always. Schema injected after render is schema that may not be read on the crawl that matters. Emit it server-side or at build time, in a script tag of type application/ld+json in the head. This site prerenders every page for exactly this reason.
Three types cover most sites
BreadcrumbList on every page, because it changes the visible result and almost nothing else does. Organization once, on the homepage, so the entity is unambiguous. Then whichever single type describes the page: Article, Product, SoftwareApplication, FAQPage. Adding more types than the page honestly is does not help and creates work to maintain.
Measured, not asserted
A whole-site scan of this project found 22 distinct types across 28 pages with zero parse failures. The value of that scan is the pages it would have flagged, not the ones it passed.
Free tool for this: Schema Inventory. No account, nothing uploaded.
Where this goes deeper
Every number on this page comes from one complete dataset: one product taken from zero to 100K+ users on search alone, with nothing spent on advertising. The full argument is Chapter 7 of the book. Five chapters are free to read.